Video clamping circuit with variable clamping level during blanking

ABSTRACT

Unclamped video signals are applied to the first plate of a capacitor, the second plate of which is discharged by a resistor to ground, the discharge being limited by a clamping diode. The clamping diode is connected between the capacitor and a resistor, which in turn is connected to a first voltage supply. A second diode, normally forward biased, connects the clamping diode to a second supply, thereby normally applying to the clamping diode the voltage of the second supply plus a forward diode drop. A transistor has its collector-emitter circuit connected across the second diode; a normally closed switching means bridges the collector and base of the transistor. During video blanking intervals, the second diode is reverse biased and the effective diode of the transistor is energized, thereby applying to the clamping diode the voltage of the second supply minus a forward diode drop. During graphics mode transmission, the switching means is opened, thereby applying to the clamping diode, during blanking, the voltage of the second supply minus the collectoremitter drop of the transistor, which is saturated.

United States Patent [191 Habib [451 May 14, 1974 VIDEO CLAMPING CIRCUIT WITH VARIABLE CLAMPING LEVEL DURING BLANKING Nissin I-labib, Neptune, NJ.

[73] Assignee: Bell Telephone Laboratories Incorporated, Berkeley Heights, NJ.

22 Filed: d Dec. 15, 1972 [2]] Appl. No.: 315,491

[75] lnventor:

[52] US. Cl. 307/237, l78/7.l DC, 307/264 Oda 307 237 Primary Examiner iohn Zazworsky Attorney, Agent, or Firm--Daniel D. Dubosky UNCLAMPED VIDEO IN [57] ABSTRACT Unclamped video signals are applied to the first plate of a capacitor, the second plate of which is discharged by a resistor to ground, the discharge being limited by a clamping diode. The clamping diode is connected between the capacitor and a resistor, which in turn is connected to a first voltage supply. A second diode, normally forward biased, connects the clamping diode to a second supply, thereby normally applying to the clamping diode the voltage of the second supply plus a forward diode drop. A transistor has its collectoremitter circuit connected across the second diode; a normally closed switching means bridges the collector and base of the transistor. During video blanking intervals, the second diode is reverse biased and the effective diode of the transistor is energized, thereby applying to the clamping diode the voltage of the second supply minus a forward diode drop. During graphics mode transmission, the switching means is opened,

thereby applying to the clamping diode, during blanking, the voltage of the secondsupply minus the collector-emitter drop of the transistor, which is saturated.

5 Claims, 2 Drawing Figures BLANKING PULSE VIDEO OUT BACKGROUND OF THE INVENTION This invention relates to video signal processing. More particularly, it relates to video clamping circuitry.

The term clamping" refers to a class of DC restoration operations in which either the positive or the negative extremities of a time variant signal are drawn to a given reference potential known as a clamping level. One standard method of accomplishing this is to couple the signal to a first terminal of a capacitor, the other terminal of which is connected to ground through a resistor. Thus, the grounded resistor tends to discharge the second plate. Limits are placed upon this discharge by addition of a clamping diode which connects the second plate to a reference potential. Whenever the second plate drops low enough to forward bias the clamping diode, further discharge is prevented. Thus, the extremities of the signal are clamped to the reference potential afforded to the clamping diode.

In video systems, it has been found that substantial subjective performance advantage is achieved if each line contains at least a minimal amount of blackness. Thus, ifa given video line has no blackness, but instead only has intermediate tones of gray, it is desirable to clamp the darkest portion of the line to black, with the remainder of the signal being shifted proportionally. In this fashion, a subjectively pleasing picture is assured.

The earliest approach to video clamping features simply the clamping of the active portion of video lines to a reference, usually zero volts, which correspond to black, but to do no clamping during the blanking inter vals. Subsequently, it was found that such clamping operations for certain classes of signals having little tonal contrast brought about camera saturation problems, due to interaction with gain control circuitry, and a second clamping level was established during the blanking interval. Since camera output during blanking is a fixed voltage, the second clamping level afforded an absolute limit to the amount of DC signal displacement which could be done and gain related saturation problems were obviated. Later still, a class of improvement circuits was developed which featured an adjustable ,clamping level during blanking. These circuits, which form the basis for a copending application of D. E. Lynn, Ser. No. 315,490, filed on Dec. 15, 1972, are based on the proposition that certain classes of graphics information such as pencil writing have optical characteristics which severely limit the amount of tonal contrast apparent to a scanning camera. The shifting of the clamping level during blanking sets a more severe limit to the amount of DC signal displacement which is brought about by clamping.

SUMMARY OF THE INVENTION The present invention involves the type of clamping such as that afforded by the circuits which form the basis of the aforementioned D. E. Lynn patent application. More particularly, the present invention involves a circuit arrangement whereby three separate clamping levels are provided for video signals, a first one during the active scanning, a second one, which is normally operative during blanking, and a third level intermediate the other two which is operative during blanking when the camera is scanning graphics type information.

The present invention achieves these three clamping levels by the variation of potential levels applied to a clamping diode. This variation is achieved by establishing one of three separate circuit conditions between a reference voltage source and the clamping diode. In response to three separate video conditions (i.e., active. normal blanking. or graphics mode blanking), three circuit configurations are established: a forward-biased diode is connected such that its forward drop is combined additively with the voltage of the reference source, a forward-biased diode is arranged such that its drop is combined subtractively with the voltage of the reference source, or the collector'to-emitter voltage drop of a saturated transistor couples the reference voltage source to the clamping diode.

In an illustrative embodiment of the present invention, a reference voltage source is coupled to a clamping diode by means of a second diode, the collectoremitter circuit of a transistor is connected across the second diode, and a normallyclosed switch bridges the collector and base of the transistor. During the active scan, the transistor is turned off and the second diode is forward biased such that the voltage of the reference source plus the forward drop of the second diode is coupled to the clamping diode. During blanking in normal transmission, the second diode is reverse biased and sufficient potential is applied to the transistor to turn its base-emitter junction on. Since the switch is closed at this time, however, the transistor merely represents an effective diode, and the voltage of the reference source minus the forward drop of the effective diode is coupled to the clamping diode. Finally, during blanking in the graphics transmission mode, the second diode is reverse biased and the transistor remains on, but the switch which bridges the collector-base junction of the transistor is opened. In this situation, the reference potential applied to the clamping diode is the voltage of the reference source less the collectoremitter drop of a saturated transistor.

It is a feature of the present invention that multiple mode clamping is afforded in an extremely simple and straightforward fashion. Use of multiple mode video clamping is thereby rendered quite inexpensive,

thereby enhancing video signal synthesis both from ,a

technical and from an economic point of view.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows an illustrative embodiment of the present invention which uses NPN transistors; and

FIG. 2 shows a corresponding embodiment which utilizes PNP transistors.

DETAILED DESCRIPTION signed the magnitude of 12 volts, is the reference which is utilized to provide reference potentials to the clamping diode 102. Thus, the embodiment of FIG. 1 is designed to clamp the negative extremities of the video signal to a voltage which is in range of +12 volts. The unclamped video signals areapplied to a capacitor 101. Since the other side of capacitor 101 is connected through a resistor 110 to ground, and since the video signals charge the capacitor 101 to a positive voltage, the effect of the path to ground through the resistor 110 is to attempt to discharge plate 1101 of capacitor 101. Unchecked, this would result in the removal of the DC component from the video signals.

The removal of the entire DC component is prevented, however, by the clamping diode 102. lt'may readily be appreciated that, when the voltage across resistor 110 drops more than 0.6 volt below that of node 106, diode 102 is forward biased and current flows through the diode 102 onto plate 1101 of capacitor 101. This occurs until the voltage across resistor 110 is more positive than 0.6 volt below the potential of node 106. Thereupon, diode 102 is reverse biased.

In summary, the combination of the capacitor 101 with a grounded resistor 110 results in the unclamped video input signals being shifted toward zero volts. A limitation upon this voltage shift occurs, however, by means of the clampingdiode 102. Whenever the voltage on plate 1101 of capacitor 101is lowered to the extent that diode 102 is forward biased, further shifting is prevented. Thus, the clamped video output signals, present at terminal 103, are identical to the unclamped video signals,-but with their negative extremities being clamped to a voltage, the magnitude of which is proportional to the potential of node 106.

The principles of the present invention provide for the variation of potential at node 106 in response to a variety of video conditions. Normally (i.e., during the active scan in normal face-to-face transmission), transistor 112, which is energized only by video blanking pulses, is turned off. Similarly, a normally closed switch 109 which is opened whenever the graphics mode is utilized, remains closed. During this time, current flows from a 17 volt source 113 through a resistor 114 and through forward-biased diode 107. At such time, the base-emitter diode of NPN transistor 118 is reverse biased. Thus, during the active scan, diode 107 is forward biased and the potential at node 106 is equal to +l2.6 volts, the potential of the source 108 plus the forward drop of diode 107. Due to the additional forward drop of diode 102 when clamping occurs, the voltage to which the video signals are clamped is +12 volts.

Whenever a blanking interval occurs, a pulse is applied to the base of transistor 112 so it conducts and goes into saturation. This causes its collector to drop nearly to ground, and thereby to forward bias the baseemitter junction of transistor 118 and to reverse bias diode 107. Under this circumstance, current is drawn both from the 12 volt source 108 and from the 17 volt source 113. Thus, the clamping potential of node 106 during this time is equal to +1 1.4 volts, the voltage of source 108 minus the forward drop of the base-emitter diode of'transistor 118. The actual voltage at which the video is clamped is therefore 10.8 volts.

Whenever the graphics mode is utilized, normally closed switch 109 is opened, and transistor 118 no longer functions simply as a diode. During the actively scanned region, since transistor 112 remains in an off condition, transistor 118 is likewise turned off and the reference potential of node 106 is maintained at 12.6 volts due to the forward biasing of diode 107. Whenever a video blanking pulse occurs, however, transistor 118 not only conducts, but is driven into saturation. In this situation, the reference potential at node 106 is equal to the 12 volt potential of source 108 less the collector-to-emitter drop of the saturated transistor 118, or approximately +1 1.7 volts. During this time, therefore, the actual voltage at which the video is clamped is 11.1 volts.

In summary, during the active scan whenever graphics mode is not being utilized, the video input signals are clamped at +12 volts since diode 107 is then forward biased and since neither of transistors 118 and 112 are conducting. During blanking when the graphics mode is not being utilized, the clamping voltage is 10.8 volts, since diode 107 is reverse biased, transistor 112 is saturated, and the base-emitter diode of transistor 118 is forward biased. During the graphics mode, the clamping voltage is altered, but only during the blanking intervals. Then, diode 107 is reverse biased and transistors 118 and 112 are saturated. This affords a clamping voltage of l 1.1 volts.

It is apparent that the clamping levels afforded by the embodiment of FIG. 1 may be varied in accordance with particular needs by altering magnitudes of select ones of the component values. For example, alteration of the potential of source 108 similarly changes the effective reference potentials supplied to the clamping diode 102 at node 106.

Whereas the embodiment of FIG. 1 utilized positive voltage supplies and NPN transistors, it is clear that the principles of the present invention are by no means limited thereto. Forexample, FIG. 2 shows a schematic diagram in which the voltage supplies are negative in polarity, the diodes 202 and 207 are reversed from their counterparts in FIG. 1, and transistors 204, 212, and 218 are PNP transistors. For such an embodiment, a negative video blanking pulse is applied to energize transistor 212. Otherwise, the operation of the embodiment of FIG. 2 is the same as that of FIG. 1 with the operation of each element in FIG. 2 being identical to the operation of the element in FIG. 1 having the same units and tens digits in its identifying number.

It is therefore to be understood that many equally obvious variations may occur to those skilled in the art without departing from the spirit or the scope of principles of the present invention.

What is claimed is:

1. Apparatus for providing a variable clamping diode comprising:

a reference voltage source;

a second diode connecting said clamping diode to said reference voltage source;

means for normally maintaining said second diode in a forward-biased condition;

a first transistor, normally non-conducting and having its collector and emitter terminals connected across said second diode;

normally closed switching means between the collector and base terminals of said first transistor; and

potential to a means responsive to a control signal for simultaneously reverse biasing said second diode and for energizing said first transistor;

whereby the potential established at said clamping diode during the interval when said control signal energizes said first transistor is dependent on the status of said normally closed switching means.

2. Apparatus as described in claim 1 wherein said means for simultaneously reverse biasing said second diode and for energizing said first transistor includes:

a second transistor having its collector connected to the emitter of said first transistor; and

means for applying the control signal across the baseemitter junction of said second transistor.

3. Apparatus as described in claim 1 wherein said means for maintaining includes:

a second voltage source having a magnitude greater than that of said reference voltage source; and

a resistor connected between said second voltage source and the junction of said second diode with said clamping diode.

4. A circuit for providing three reference potentials to a clamping diode comprising first and second sources of potential with respect to ground, a second diode having one of its electrodes connected to said second source and its other electrode connected to said clamping diode, means for connecting said first source to the other electrode of said second diode whereby a first reference potential is provided at said other electrode when said second diode is forward biased, a transistor having base, emitter and collector electrodes, normally closed switching means for connecting together said base and collector electrodes thereby forming an effective diode, means for directly connecting the collector and emitter electrodes to opposite electrodes of said second diode such that the effective diode is oppositely poled to said second diode, means for selectively coupling said emitter electrode to ground whereby said effective diode is forward biased and a second reference potential is developed at the other electrode of said second diode, and means for energizing the base electrode of said transistor whereby said transistor is driven into saturation when said normally closed switching means is opened thereby resulting in a third reference potential at the other electrode of said second diode.

5. A video clamping circuit comprising: a capacitor; means for applying unclamped video signals to a first plate of said capacitor;

a first resistor connected between the second plate of a clamping diode having a first electrode connected to the second plate of said capacitor. said clamping diode tending to limit the discharge of the second plate of said capacitor in response to a voltage of a second electrode of said clamping diode;

first and second reference voltage sources, the magnitude of said first source being greater than the magnitude of said second source;

a second resistor connected between said first reference voltage source and the second electrode of said clamping diode;

a second diode, normally forward biased, for connecting said second reference voltage source to the second electrode of said clamping diode, thereby normally applying to said clamping diode the voltage of said second reference voltage source plus the voltage drop of a forward-biased diode;

a transistor, normally non-conducting, having its collector-emitter circuit connected across said second diode;

normally closed switching means for bridging the collector and base electrodes of said transistor; and

means, responsive to video blanking pulses, for energizing the base-emitter diode of said transistor and for simultaneously reverse biasing said second diode, thereby applying to said clamping diode the voltage of said second reference voltage source minus the voltage drop of a forward-biased diode;

the collector-to-emitter voltage of said transistor. 

1. Apparatus for providing a variable potential to a clamping diode comprising: a reference voltage source; a second diode connecting said clamping diode to said reference voltage source; means for normally maintaining said second diode in a forwardbiased condition; a first transistor, normally non-conducting and having its collector and emitter terminals connected across said second diode; normally closed switching means between the collector and base terminals of said first transistor; and means responsive to a control signal for simultaneously reverse biasing said second diode and for energizing said first transistor; whereby the potential established at said clamping diode during the interval when said control signal energizes said first transistor is dependent on the status of said normally closed switching means.
 2. Apparatus as described in claim 1 wherein said means for simultaneously reverse biasing said second diode and for energizing said first transistor includes: a second transistor having its collector connected to the emitter of said first transistor; and means for applying the control signal across the base-emitter junction of said second transistor.
 3. Apparatus as described in claim 1 wherein said means for maintaining includes: a second voltage source having a magnitude greater than that of said reference voltage source; and a resistor connected between said second voltage source and the junction of said second diode with said clamping diode.
 4. A circuit for providing three reference potentials to a clamping diode comprising first and second sources of potential with respect to ground, a second diode having one of its electrodes connected to said second source and its other electrode connected to said clamping diode, means for connecting said first source to the other electrode of said second diode whereby a first reference potential is provided at said other electrode when said second diode is forward biased, a transistor having base, emitter and collector electrodes, normally closed switching means for connecting together said base and collector electrodes thereby forming an effective diode, means for directly connecting the collector and emitter electrodes to opposite electrodes of said second diode such that the effective diode is oppositely poled to said second diode, means for selectively coupling said emitter electrode to ground whereby said effective diode is forward biased and a second reference potential is developed at the other electrode of said second diode, and means for energizing the base electrode of said transistor whereby said transistor is driven into saturation when said normally closed switching means is opened thereby resulting in a third reference potential at the other electrode of said second diode.
 5. A video clamping circuit comprising: a capacitor; means for applying unclamped video signals to a first plate of said capacitor; a first resistor connected between the second plate of said capacitor and ground, said first resistor tending to discharge the second plate of said capacitor; a clamping diode having a first electrode connected to the second plate of said capacitor, said clamping diode tending to limit the discharge of the second plate of said capacitor in response to a voltage of a second electrode of said clamping diode; first and second reference voltage sources, the magnitude of said first source being greater than the magnitude of said second source; a second resistor connected between said first reference voltage source and the second electrode of said clamping diode; a second diode, normally forward biased, for connecting said second reference voltage source to the second electrode of said clamping diode, thereby normally applying to said clamping diode the voltage of said second reference voltage source plus the voltage drop of a forward-biased diode; a transistor, normally non-conducting, having its collector-emitter circuit connected across said second diode; normally closed switching means for bridging the collector and base electrodes of said transistor; and means, responsive to video blanking pulses, for energizing the base-emitter diode of said transistor and for simultaneously reverse biasing said second diode, thereby applying to said clamping diode the voltage of said second reference voltage source minus the voltage drop of a forward-biased diode; whereby the video signals at said second plate of said transistor are clamped to a first level during the active video interval, to a second level during video blanking in normal transmission, and to a third level during video blanking when said normally closed switching means is opened and the second electrode of said clamping diode is provided with the voltage of said second reference voltage minus the collector-to-emitter voltage of said transistor. 